The present invention relates to a decompression device for a four-stroke engine, and more particularly, to a decompression device which temporarily opens the exhaust valve before the piston reaches the top dead center (TDC) in the compression stroke, when starting a hand-held four-stoke engine.
The engines are frequently used in mowers, chain-sawing machines, pumps and exhaust fans etc. As a two-stroke engine is apt to cause air pollution, it is gradually substituted by a four-stroke engine which serves as the power source of the afore-mentioned machines.
An inlet valve and an exhaust valve are used to control the intaking and exhausting of the engine, respectively. The inlet valve and the exhaust valve are opened and closed by using a cam-follower mechanism, thereby realizing an inlet stroke, a compression stroke, a power stroke (or combustion stroke) and an exhaust stroke.
FIG. 1 is a front view illustrating a cam-follower mechanism of a conventional four-stroke engine. An arrow indicates the rotating direction of the cam. FIG. 2A is a front view illustrating the cam-gear portion shown in FIG. 1. FIG. 2B is a side view illustrating the cam-gear portion shown in FIG. 1.
As shown in FIG. 1, the cam-follower mechanism comprises a cam 101 having a profile surface and a central hole 107, a gear 102 having the same central hole 107 as that of cam 101 and is fixed to cam 101, a first follower 103 and a second follower 105 being biased, respectively, by a spring (not shown) so as to keep in contact with cam 101 and swingable around a pin 108, and a first link 104 and a second link 106 keeping in contact with first follower 103 and second follower 105 at a point M and a point N, respectively.
Hereinbelow, the operation of the cam-follower mechanism will be described.
Referring to FIG. 1, gear 102 is driven to drive cam 101 to rotate clockwise. First follower 103 and cam 101 are constantly kept in touch with each other as described above. When first follower 103 is in contact with the ascending profile of cam 101, first follower 103 is lifted by cam 101 and a counterclockwise pivoting moment around pin 108 is generated. Thus, the point M rises and pushes up first link 104 and, in turn, further opens an inlet valve (not shown) through another mechanism (not shown).
Next, when first follower 103 is in contact with the descending profile of cam 101, the inlet valve is closed by the restoring force of an inlet valve spring (not shown).
Likewise, when second follower 105 is kept in contact with the ascending profile of cam 101, second follower 105 is lifted by cam 101 and a clockwise pivoting moment around pin 108 is generated. Thus, the point N rises and pushes up second link 106 and, in turn, further open an exhaust valve (not shown) through still another mechanism (not shown).
Next, when second follower 105 is in contact with the descending profile of cam 101, the exhaust valve is closed by the restoring force of an exhaust valve spring (not shown).
It should be understood that first follower 103 and second follower 105 are located on different two planes parallel to the paper plane of FIG. 1.
FIG. 3 is a chart showing the relationship between the opening xcexa8 of the inlet (exhaust) valve and the crank angle xcex8 in a conventional four-stroke engine. As shown in FIG. 3, the horizontal and vertical axis denote the crank angle xcex8 and the opening xcexa8 of the valves respectively, the symbols E, I, C and P denote the exhaust stroke, the inlet stroke, the compression stroke and the power stroke respectively, and the symbols TDC and BDC denote the top dead center and the bottom dead center of the engine piston respectively.
Referring to FIG. 3, the curve shown by dotted lines designate the relationship between the opening xcexa8 of an inlet valve IV and the crank angle xcex8, while the curve shown by solid lines designate the relationship between the opening xcexa8 of an exhaust valve EV and the crank angle xcex8. When the crank angle xcex8 is from 0 to 360xc2x0, the piston moves from a bottom dead center (BDC) to a top dead center (TDC), and then return to the BDC.
Exhaust stoke E is realized when the crank angle xcex8 is approximately between 0xc2x0 and 180xc2x0. In this duration, exhaust valve EV is gradually opened to a maximum opening and then gradually closed. On the other hand, inlet stroke I is realized when the crank angle xcex8 is approximately between 180xc2x0 and 360xc2x0. In this duration, inlet valve IV is gradually opened to a maximum opening and then gradually closed. Thereafter, a compression stroke C is realized when the crank angle xcex8 is approximately between 360xc2x0 and 540xc2x0, and a power stroke P is realized when the crank angle xcex8 is approximately between 540xc2x0 to 720xc2x0. Among these four strokes, the pressure in the cylinder is the maximum when the piston reaches the TDC in the compression stroke C. Therefore, the resistance force of the engine shaft is the maximum.
When starting a conventional four-stroke engine, an external force (such as electric force or human force) is required to rotate the engine shaft so as to provide a moment of inertia for the engine to begin self-running after ignition in the combustion chamber. Due to the maximum resistance force when the piston reaches the TDC in the compression stroke, a larger force is required to overcome it so as to start the engine.
As a result, if the exhaust valve is opened slightly and temporarily before the piston reaching the TDC in the compression stroke, it is helpful for decreasing the pressure in the cylinder and reducing the starting force. However, this function has to be disabled when the engine is operated normally so that the efficiency is not affected. It is therefore a problem to be solved by the present invention.
In order to solve the above problem, it is therefore an object for the present invention to provide a decompression device for a four-stroke engine which can decompress the pressure in the cylinder by opening the exhaust valve before the TDC in the compression stroke, thereby reducing the starting force. Alternatively, this device can also disable the decompression function when the engine is operated normally.
In accordance with the present invention, there is provided a decompression device for a four-stroke engine, wherein the operating cycle of the engine includes an inlet stroke, a compression stroke, a power stroke and an exhaust stroke, and the engine includes a piston which reaches a top dead center (TDC) and a bottom dead center (BDC) at the two ends of each stroke, the decompression device comprising: an inlet valve and an exhaust valve for controlling the inlet stroke and the exhaust stroke respectively, a first rocker and a second rocker for driving the inlet valve and the exhaust valve respectively, a first link and a second link for driving the first and the second rocker respectively, a first follower and a second follower for driving the first link and the second link respectively, and a cam having a cam profile surface for driving the first and the second followers, characterized in: that the cam further comprises a guiding recess provided on the side surface thereof and a slot provided on the cam profile surface communicating with the guiding recess; that the decompression device further comprises a centrifugal member, received within the guiding recess, having a projecting portion and being rotatable and movable within the guiding recess, and a spring having two ends fixed to the cam and the centrifugal member; and that through the cooperation with the spring and the guiding recess, when the cam is rotated at a lower speed, the projecting portion of the centrifugal member projects through the slot to the outside of the cam profile surface under the action of a weaker centrifugal force, thereby lifting the second follower so as to open the exhaust valve before the TDC of the compression stroke, and when the cam is rotated at a higher speed, the projecting portion of the centrifugal member sinks into the inside of the cam profile surface under the action of a stronger centrifugal force, thereby closing the exhaust valve.
In the afore-described decompression device, preferably the centrifugal member further comprises an elliptic through hole, and the cam further comprises at least one pin located in the guiding recess and penetrating through the elliptic through hole, thereby guiding the movement and rotation of the centrifugal member.